The goal is to analyze the structure and function of clusters of tandem repeats of ribosomal DNA, including both the 18S-28S rDNA and 5S rDNA. Using pulse field gels, attempts would be made to fractionate the complete DNA contents of nucleoli intact and assign them to various chromosomal nucleolar organizers. The major 5S rDNA cluster from chromosome 1 would be similarly analyzed. Clones containing parts of the 50 kb 18S-28S rDNA (and others with several 5S rDNA repeats) are already available. Now YAC clones will be sough that have the maximum number of tandem repeats. Both the heterogeneity within a block of repeats and the nature of junction sequences at the edge of blocks will be investigated, and can contribute to the understanding of how repeat units are organized and can vary. For the 185-285 rDNA, the availability of cloned complete repeat units will make possible systematic studies of the effects of site-specific deletions on the function of rDNA. Function will be studied after transfection into cells. Particular attention will be focused on determining how non-transcribed spacer sequences may help organize functional nucleoli; this may provide model information for comparable features of all genes.